1. Field of the Invention
The present invention relates to a base station apparatus and frame synchronization acquisition method in a radio communication system in which frame synchronization is acquired between base station apparatuses.
2. Description of the Related Art
In radio communication systems, in order to cancel interference between base station apparatuses to increase the system capacity, as disclosed in Japanese Unexamined Patent Publication HEI5-308333, a method is sometimes adopted in which the frame synchronization is acquired between a plurality of base station apparatuses, and the timing of a transmission slot and that of a reception slot are made the same between the base station apparatuses.
FIG. 1 is a diagram illustrating a radio communication system including conventional base station apparatuses. It is assumed that base station apparatus 12 synchronizes the frame timing thereof with the frame timing of base station apparatus 11 which is currently a reference of the frame synchronization. It is further assumed that base station apparatus 11 performs communications with mobile station apparatuses 21 and 23 existing in communication area 31 of base station apparatus 11, base station apparatus 12 performs communications with mobile station apparatus 22 existing in communication area 32 of base station apparatus 12, and that base station apparatus 13 starts communicating.
Base station apparatus 13 acquires the frame timing of base station apparatus 11 which is the reference of the frame synchronization before starting ordinary communications. In this case, since base station apparatus 11 exists out of communication area 33 of base station apparatus 13, base station apparatus 13 cannot receives a signal from base station apparatus 11 directly.
Therefore, base station apparatus 13 synchronizes the frame timing thereof with that of base station apparatus 12 to be indirectly in frame synchronization with base station apparatus 11. Base station apparatus 13 starts communicating with mobile station apparatus 24 existing in communication area 33 thereof after acquiring the frame synchronization.
Thus, in the case where the conventional base station apparatus detects the frame timing of a neighboring base station apparatus to synchronize, the conventional base station apparatus detects the frame timing of the neighboring base station apparatus existing closest thereto, and performs communications in synchronization with the frame timing of such a neighboring base station apparatus.
FIG. 2 is a diagram illustrating the frame timing of each base station apparatus after the frame synchronization acquisition is completed.
As illustrated in FIG. 2, due to errors caused by that each base station apparatus receives a transmission signal from another base station apparatus to detect the timing, the transmission timings of frames to mobile station apparatuses from respective base station apparatuses are not completely synchronous, resulting in errors almost equivalent to a guard time.
Since the error of the transmission timing between base station apparatuses 11 and 12 is within a range of the guard time, and both base station apparatuses do not transmit a message at the guard time portion, interference does not occur between both base stations. Similarly, the interference does not occur between base station apparatuses 12 and 13.
However, the error of the frame synchronization between the base station apparatuses are accumulated, and consequently some part of the transmission signal of base station apparatus 11 and that of base station apparatus 13 overlap in time each other, causing the case where the interference sometimes occurs between base station apparatuses 11 and 13.
Such interference occurs at a portion with oblique lines in FIG. 1 where the communication area of base station apparatus 11 and that of base station apparatus 13 overlap each other. Accordingly, as illustrated in FIG. 2, the interference occurs when base station apparatus 13 transmits a signal to mobile station apparatus 24 in some frame, and base station apparatus 11 transmits a signal to mobile station apparatus 11 in next frame.
Thus, in the conventional base station apparatuses, when a base station apparatus with the frame timing as a reference of the frame synchronization does not exist in the communication area of some apparatus, the apparatus does not acquire the frame synchronization directly, but acquires that indirectly, resulting in the problem that errors of frame timings between the base station apparatuses become large, and that the interference occurs.